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6 Difficult Concepts You Must Know for HSC Biology Exam

In this post, our Science Team explain 6 difficult concepts you must know for the HSC.

There are 6 difficult HSC Biology concepts students struggle with in the HSC Biology exam. Often this is caused by confusing two similar concepts. Make sure you have the following 6 HSC Biology concepts clarified!

HSC Biology Concept 1: Difference Between Amino Acids, Polypeptides and Proteins

Students often get amino acids, polypeptides and proteins confused. It is important to understand these terms in order to understand translation and the experiments of Beadle and Tatum.

Amino acids form the building blocks of the proteins in our body. They are composed of different arrangements of Carbon Hydrogen Oxygen and Nitrogen (CHON). Some amino acids are absorbed from our food (essential) and others need to be synthesised (non-essential).

Recall that amino acids are joined together by peptide bonds during the translation process to form polypeptides.

Blog Biology 6 Difficult Concepts to Master for the HSC Biology Exam Amino Acids 1 Blog Biology 6 Difficult Concepts to Master for the HSC Biology Exam Amino Acids 2

Individual amino acids are joined together to form polypeptides. Proteins are composed of one or more polypeptides that have been joined together and given a 3D configuration. Until they have the correct configuration, proteins will not be ‘functional’.


HSC Biology Concept 2: The Difference Between DNA Replication and Transcription/Translation

Students can get these confused as they both involve unzipping of the DNA in the first stage, but the end products are very different.

Blog Biology 6 Difficult Concepts to Master for the HSC Biology Exam DNA Replication

DNA replication only occurs in preparation for cell division (meiosis or mitosis). The DNA is unzipped by helicase, then each half of the DNA strand is used as a template. Complementary nucleotides are added by DNA polymerase and the end product is two identical copies of a chromosome.

Transcription and Translation


Transcription and translation (protein synthesis) happens during interphase. Inside the nucleus, DNA is unzipped by helicase, but only at the location of a specific gene. A single strand of complementary mRNA is produced by RNA polymerase and then the mRNA leaves the nucleus. Transcription is complete.

In the cytoplasm a ribosome ‘reads’ the mRNA. tRNA molecules enter the ribosome and drop-off amino acids. The ribosome joins the amino acids together using peptide bonds, the end product is a polypeptide chain.

DNA replication
Transcription and translation
Only before cell division During interphase
End product
Two identical copies of a chromosome A polypeptide
Units of end product
Nucleotides Amino acids
In the nucleus Transcription in the nucleus then translation in the cytoplasm

HSC Biology Concept 3: Beadle and Tatum

Remember: all enzymes are proteins, but not all proteins are enzymes! 

In order to understand the experiments conducted by Beadle and Tatum, students must recall that enzymes can catalyse reactions to change one product into another.  In this way, they can be used to synthesise new amino acids from existing ones.

Blog Biology 6 Difficult Concepts to Master for the HSC Biology Exam Beadle and Tatum

Beadle and Tatum’s experiment used a bread mould that needs the amino acid Arginine to grow. If Arginine isn’t available in the immediate environment the bread mould can synthesise Arginine from the amino acid Glutamate using four different enzymes. This is referred to as the Arginine synthesis pathway.

Blog Biology 6 Difficult Concepts to Master for the HSC Biology Exam Beadle and Tatum 2

At the time, the Arginine synthesis pathway was well understood, but the role of DNA in this pathway was unknown. Beadle and Tatum had a hypothesis that active DNA segments called genes coded for each enzyme.

To test this hypothesis they exposed many samples of the bread mould to radiation in order to cause mutations of its DNA. Different samples of bread mould ended up with different mutations.

In some samples the bread mould didn’t grow as the mutations had interrupted the Arginine synthesis pathway. For example, in some samples it was found that the Arginine synthesis pathway was stopping at Citruline. However, when the bread mould was supplied with Arginosuccinate in the growth medium it could synthesise Arginine and grow normally.

This indicated to Beadle and Tatum that the bread mould could synthesise Citruline, but not Arginosuccinate (and therefore not Arginine).  The radiation had caused a mutation on the gene for Enzyme 3 on the Arginine synthesis pathway.

Blog Biology 6 Difficult Concepts to Master for the HSC Biology Exam Beadle and Tatum 3

By producing strains of bread mould with different mutations, Beadle and Tatum could repeat the process to identify the gene for each of the four enzymes.

The one gene, one enzyme (or protein) hypothesis was later changed to the one gene, one polypeptide hypothesis when it was discovered that each gene codes for an individual polypeptide and that multiple genes can be involved in coding for an enzyme (or protein).


HSC Biology Concept 4: The difference Between Whole Organism Cloning and Gene Cloning

Whole organism cloning typically involves taking an unfertilised egg cell from one individual and removing the nucleus. A nucleus is then taken from a somatic cell (body cell) of a different individual and inserted into the egg cell. The egg is stimulated (to mimic fertilisation) and the egg cell should start to divide and form an embryo.

The embryo is then implanted in a surrogate mother. The resulting offspring will only have physical characteristics of the individual that donated the nucleus, as this is its only source of DNA. The typical example of this is Dolly the sheep.

ATTACHMENT DETAILS Blog-Biology-6-Difficult-Concepts-to-Master-for-the-HSC-Biology-Exam-Cloning

In gene cloning a gene of interest is identified, often on the human genome, and cut out using restriction enzyme. The DNA (plasmid) of a bacterium is then cut open (using restriction enzyme again) and the gene is inserted into the plasmid using DNA ligase. The bacteria is then encouraged to multiply resulting in many copies of the gene of interest.

The gene may be multiplied so it can be inserted into a different organism later (e.g. a plant), or the gene may be expressed by the bacteria (via transcription and translation) so that it produces a useful product (e.g. insulin). Some students forget to include the role of plasmids and bacteria when describing gene cloning.


HSC Biology Concept 5: The difference Between Antibodies and Antigens

Antigens are proteins found on the surface of pathogens such as bacteria or viruses that enter the body.

Antibodies are produced by B-Cells and bind to the antigens helping to disable the pathogen and mark it for destruction.

Antibodies are produced by your body, antigens (antibody generators) are from outside of the body!


Unfortunately antigens are found on some substances that are not actually pathogens such as pollen or transplanted organs.


HSC Biology Concept 6: Antigens and Organ Transplants

Your organs have antigens on their surface. Your immune system recognises these as part of your body and doesn’t launch an immune response against them.

However, organs or parts of organs can be transplanted from the body of a donor into the body of a patient.

The donated organ will have surface antigens unique to the donor and so the patient’s body recognises these as foreign. The immune system will attack these foreign cells and try to destroy the donated organ.

The patient is given immune suppressants to stop the body from attacking the donated organ, but this leaves the patient more susceptible to actual pathogens that cause illness. The patient will have to remain on immune suppressants for the rest of their life.



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Written by Matrix Education

Matrix is Sydney's No.1 High School Tuition provider. Come read our blog regularly for study hacks, subject breakdowns, and all the other academic insights you need.


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